Cream together butter and sugar in a large bowl until smooth. Add egg yolks and beat until light. In a separate bowl, sift flour, baking powder, and salt together. Add dry ingredients alternately with lemon juice to the butter mixture. Stir after each addition but do not over mix. Fold in stiffly beaten egg whites, poppy seeds and lemon peel. Spoon batter into prepared muffin tins and bake at 375 degrees for 20 - 25 minutes.